1. Field of the Invention
This invention relates generally to an electrochromic system and, more particularly, to an electrochromic system for controlling the color of electrochromic panes.
2. Discussion of Background
An electrochromic system is a system in which a coloration state can be modified under the effect of an electric field. It is particularly applicable to control the amount of solar gain in buildings or the passenger compartments of automobiles.
A system of this type uses a film of an electrochromic material, that is to say a material capable of inserting cations such as protons or lithium ions, in a reversible manner. The oxidation states corresponding to the two states, inserted and de-inserted, have different coloration states, including in the case of panes a transparent decolored state.
For this insertion or de-insertion to take place, it is necessary to provide, alongside the film of electrochromic material, a source of cations and a source of electrons, made respectively of a film of an electrolyte having ion conductivity and of an electrically conducting film. Furthermore, the system includes a counter-electrode, which is also capable of inserting and de-inserting cations in a reversible manner, and symmetrically relative to the film of electrochromic material. This counter-electrode is, preferably, also an electrochromic material which in the case of an electrochromic pane includes two different coloration states. This counter-electrode is chosen so that the two materials shall be simultaneously decolored.
For this counter-electrode, the cation source is also the electrolyte film while the electron source is made of a second electrically conducting film. The two electrically conducting films form two electrode, across which a potential difference is applied.
This potential difference should be sufficiently high in absolute value for the insertion and de-insertion reactions of the cations in the films of electrochromic materials to take place. As a convention, we shall speak of positive voltage for a coloration and negative voltage for a decoloration. If, for example, a conduction system based upon lithium ions is chosen with tungsten oxide and nickel oxide as electrochromic material and counter-electrode respectively, coloration is thermodynamically possible only for a potential difference of greater than about 1 volt. Decoloration can be accelerated if a non-zero potential difference is applied. The reaction kinetics are then more rapid. However, this potential difference must in both cases, be less than the thermodynamic potentials of other parasitic reactions.
The values for this example at ambient temperature may be fixed at 2 volts for the insertion reaction of the protons into the tungsten trioxide and at -1 volt for the de-insertion.
While current is supplied to the electrochromic pane, its coloration develops as a function of time. The coloration is therefore a function of the time of passage of the electric current, that is to say the degree of coloration corresponds to the quantity of charge inserted into the electrochromic material. It is therefore possible to obtain various degrees of coloration by modulating the switching time, that is to say the amount of time that elapses during a change from one coloration state to another colored or decolored state.
Still with regard to this example, when the starting state of the electrochromic pane is known, it is only necessary to measure the charge delivered by the electrical supply circuit and, since the charge corresponding to the desired degree of coloration is known, to shut-off the supply as soon as this charge is reached. The charge can be measured by means of a current integrating circuit.
In contrast, to pass from one coloration state to another, it is necessary to store in memory the charge corresponding to this particular change and to have available a memory to contain the values of the charge quantities (or the corresponding switching times) needed to pass from any state to any other state. This is best done with the use of a microprocessor and memory map.
When the starting state for the electrochromic pane is a colored state, the situation is still more difficult, because it is necessary to take into account the quantity of charges already inserted. Now the coloration state does not necessarily correspond to the quantity of charges previously inserted, due to a phenomenon known as self-discharge of the electrochromic pane. This corresponds to a loss of charge and a return towards the decolored state. After a length of time, uncertainty exists with regard to the charge of the pane and therefore its coloration state.
It is therefore not possible to just memorize the quantity of charges that have been delivered by the electrical supply, because it is not indicative of the coloration state after a long period of time. This value cannot be taken as a starting point for a new setting, because the quantity of charges inserted will decrease.
One solution that may be envisaged for determining the starting coloration state would be to measure the light transmission and thus the degree of coloration by means of a photoelectric cell and a light source placed on either side of the pane, and preferably far from the electric supply leads.
Starting from this value of light transmission, the new setting of coloration degree is set, by generating a first voltage in the case of a coloration or a second voltage in the case of a decoloration and by modulating the switching time. This solution has the great disadvantage of requiring an extra apparatus for measuring the degree of coloration, and the necessary extra electric wiring. It should be noted that such a device may also determine when the desired light transmission is reached.